Magnetoresistive elements feature an electrical resistance that strongly depends on the magnitude and/or direction of an externally applied magnetic field. Generally, there exist a large variety of different magnetoresistive elements making use of different fundamental effects. For example, the Anistropic Magnetoresistive (AMR) effect shows a change in electrical resistance in the presence of a magnetic field. AMR sensors are typically made of a soft-magnetic material, such as nickel-iron (Permalloy) thin film deposited on a silicon wafer. The magnetoresistive effect is dependent mainly on the relative direction between an electrical current and the direction of magnetization.
Another effect is the oscillatory exchange interaction of ferromagnetic layers depending on the thickness of the normal metal spacer layer and the giant magneto resistance (GMR) in the exchange-coupled multilayers. The latter effect is driven by the relative magnetic orientation of the adjacent ferromagnetic layers. Replacing the non-magnetic layers by an insulator eliminates the exchange. However, by pinning the first ferromagnetic layer with an anti-ferromagnetic layer an anti-parallel oriented stack can be caused by an external magnetic field. Applying the electric current perpendicular through the ferromagnetic-insulating ferromagnetic stack results in a large change of the tunnel magneto resistance (TMR).
In general, the magnitude and/or direction of the magnetic field can be measured by using at least one magnetoresistive element. However, using only one element makes the device rather sensitive to externally driven drifts, such as temperature and hysteresis drifts. In order to avoid this problem, relative measurements, being sensitive just to external magnetic changes, can be performed. This can be realized, for example, in a Wheatstone bridge arrangement. At least four elements of identical resistance are required. Two adjacent pairs of elements typically exhibit different magnetoresistive behaviors. In such a design, the measured signal is proportional to the relative changes of the magneto resistance of adjacent resistance pairs.
Moreover, when implemented in combination with elements made of soft magnetic material, such as Permalloy, various components of the externally applied magnetic field can be effectively separated and separately detected, thus allowing for a determination of the direction of the magnetic field. Hence, a Wheatstone bridge arrangement serves as a universal tool to generate an electrical signal that depends on the magnitude and/or the direction of an externally applied magnetic field.
In typical implementations, such a Wheatstone bridge features an offset drift that might be caused by slight deviations of the electrical and/or magnetic properties of its components. Even magnetoresistive elements of the same type series might be subject to inevitable production tolerances. Consequently, a bridge circuit may provide a non-zero output voltage even in the absence of a magnetic field. This offset of a bridge circuit is in particular rather disadvantageous, when magnetic fields of low magnitude have to be measured, such as the earth magnetic field. In such cases the offset of a Wheatstone bridge might be in the same range as the signal that has to be measured.
Offset drift elimination can, for example, be performed by a so-called flipping procedure when AMR elements are implemented in the bridge circuit. A short magnetic pulse can be applied to the sensor element in order to reverse its sensitivity. Making use of periodically alternated flipping pulses and a lock-in amplifier, the resulting output of the bridge becomes independent of sensor and amplifier offset. This flipping procedure is only applicable to AMR elements that generally feature a limited magnetic sensitivity, and a rather large size and a relatively low electric resistance that leads to fairly high power consumption in operation mode.
In contrast to AMR elements, GMR elements feature a significantly higher magnetic sensitivity, and they are smaller in size and have a higher electrical resistance. However, the flipping procedure for offset compensation cannot be applied to GMR elements, because GMR elements do not inherently allow a reversal of their sensitivity. GMR multilayer elements feature another disadvantage in form of being insensitive to the direction of a magnetic field. Directional magnetic field sensors can therefore be implemented by making use of AMR elements or GMR spin valves that feature a spatially fixed ferromagnetic layer and a free layer, whose magnetization is free to follow an external magnetic field. In general, spin valves may be permanently destroyed by high magnetic field or current pulses. Additionally, such structures are also more sensitive to high temperatures than multilayers. Moreover, GMR spin valves require additional production processes as compared to standard, non-pinned GMR elements.
In principle, a directional sensor can be implemented on the basis of GMR multilayer elements in combination with flux guides providing spatial decomposition components of a magnetic field. Furthermore, in principle, determination of a direction of a magnetic field is possible with the above-mentioned magnetoresistive elements and configurations. However, all these approaches may suffer from temperature drift, mechanical stress or hysteresis of the magnetoresistive elements.
Even though AMR and GMR spin valve elements provide an electric resistance that depends on the direction of an external magnetic field, these elements inherently do not provide an unequivocal determination of the direction of the magnetic field. Typically, the electric resistance of those magnetoresistive elements inherently features a periodicity of 90°. An unequivocal determination of the direction of the magnetic field over a range from zero to 360° is rather sophisticated and requires, for example, an implementation of a plurality of bridge circuits.
Therefore, there is a need in the prior art for an improved offset compensation for bridge circuits that makes use of magnetoresistive elements and that provides an improved direction determination of a magnetic field. For these and other reasons, there is a need for the present invention.